1. Field of the Invention
The present invention relates to a direct-conversion frequency mixer for down converting a radio frequency signal to a based band signal, and more specifically, to a direct-conversion frequency mixer having a high linearity even at a low power source voltage.
2. Discussion of Related Art
In a general wireless communication system using radio waves, a frequency converter receives a very low power radio frequency (RF) signal through an antenna and a quadrature local oscillation (quadrature LO) signal from an internal local oscillator of the system, and converts the RF signal into a baseband frequency signal corresponding to a frequency difference between the RF signal and the quadrature LO signal. This frequency conversion can be simply achieved by a nonlinear circuit for multiplying the RF signal by the quadrature LO signal. Thus, the frequency converter generally includes a core unit used as an analog multiplier, an input/output buffer, a filter, and the like. A general theory and a prior art relating to the frequency converter circuit have been disclosed in “RF Microelectronics” by B. Razavi (Prentice Hall, 1998), and “The Design of CMOS Radio-Frequency Integrated Circuits” by Thomas Lee (Cambridge University Press, 1998).
With recent improvements in semiconductor technology, a silicon device such as a complementary metal-oxide-semiconductor (CMOS) transistor typically used in a digital VLSI chip is often applied to a radio frequency (RF) circuit. However, due to increasing integration density, a gate line width of the CMOS transistor has been reduced from 130 nm to about 90 nm while a power source voltage of a digital circuit unit has been reduced to about 1.5V or less. When different power source voltages are applied to the digital circuit unit and the RF/analog circuit unit, an additional power source conversion circuit such as a DC-DC converter is required, which increases the cost of the parts and power consumption. This is not conducive to providing consumers with the low cost and low power consumption they have been demanding. Therefore, there is a need for an RF circuit capable of being driven with a low power source voltage, like a digital circuit. Thus, a circuit with which the structure of circuits included in the RF chip may be fundamentally changed should be developed.
In the conventional RF/analog technology, more than two transistors connected in a stacked or cascade structure are used as a current bias or a current source to achieve a stable characteristic irrespective of changes in operational conditions such as process, temperature or the like. In the case of a frequency mixer circuit, a quadrature LO signal input unit 10 including quadrature LO signal conversion transistors Q1 and Q2 and an RF signal input unit 11 including an RF signal conversion transistor Q3 are connected in the stacked structure, as shown in FIG. 1. Such a Gilbert multiplier structure uses divided power source voltage for terminals, so that a voltage swing at an output terminal is low. Further, when the power source voltage is reduced to 1.5V or less, an operational voltage range is limited to an active region of a transistor, thereby lowering linearity.
In an ultra wide-band system where an in-band interference signal having a very high power level is directly input, in particular, reduction of the linearity of the frequency mixer becomes a serious problem. In the ultra wide-band system of 3.1 to 10.6 GHz, communication employs very weak radio waves of less than −41 dBm/MHz. Such an ultra wide-band system can achieve a high transmission rate of about 1 Gbps within a local area. Contrary to the existing wireless communication system in which only a certain frequency band is used and the other bands are blocked using an RF filter, an ultra wide-band transceiver receives all signals of the wide frequency band. Therefore, interference signals 30 dB or more larger than a desired signal are applied together with the desired signal to the frequency mixer, so that there arise nonlinear effects such as generation of a spurious signal due to intermodulation, degradation of sensitivity due to saturation, blocking, etc. Therefore, there is need for developing a frequency mixer having high linearity and using a low power source voltage.